Demand for the accurate analysis of optical signals continues to be of significant importance in many fields. For example, in accurate measurement, it is often important to be able to accurately sample an optical signal. The need for accurate measurement is prevalent in a wide range of fields, including optical telecommunications and optical measurement.
For example, demand for high capacity data transmission continues to grow and grow. One dominant form of transmission is optical transmission over optical fibers or the like or optical free space transmission. Current planning demands future 100 gigabytes per second (100G) systems. With such high capacity transmission systems, there is a need to demodulate a received optical signal.
One suitable encoding methodology for high bandwidth optical transmission is Differential Quaternary Phase-Shift Keying (DQPSK). In such a system, information is encoded in the phase of the transmitted signal. In particular, encoding is provided in phase changes in the transmitted signal.
One high capacity DQPSK transmission system for optical communications is Dual Polarization with Quadrature Phase Shift Keying. Example DP-QPSK systems are set out in the following:                U.S. Pat. No. 5,473,463 to Van Deventer discloses an optical receiver known as an optical hybrid device;        U.S. Patent application publication number 2007/0223932 to Hsich also discloses a coherent optical receiver device.        “Recent advances in coherent optical communication” by Guifang Li, Advances in Optics and Photonics 1, 279-307 (2009) discusses the principals of coherent optical receivers.        Other optical hybrid devices are discloses in U.S. Pat. No. 7,209,670 to Fludgerfl, U.S. Pat. No. 7,315,575 to Sunfl, U.S. Pat. No. 6,917,031 to Sunfl.        
Normally, in each of the above referenced designs, it is common to implement detection coherently by means of mixing of the electric field vectors of the aligned polarization states of an input signal and a local oscillator. A number of problems are provided with implementation of such designs. The aforementioned arrangements often rely upon interferometric structures with one arm of the interferometer providing a 90 degree phase delay. Unfortunately, the requirement for a 90 degree phase delay can often lead to difficulty in meeting tolerances. A 90 degree phase shift is equivalent to, at standard optical transmission bands, to the utilization of a 400 nanometer optical element. Allowing for say a 2 to 3 degree tolerance accuracy places about a 10 nanometer tolerance accuracy on the phased delayed element. This is difficult to provide, especially where temperature variations occur. Further, the interferometric system often leads to extremely tight tolerances on alignment. This normally leads to a high expense in manufacturing optical hybrid devices or the additional complexity of actively tuning the phase delay based upon a feedback from the signal.
As phase and amplitude detection techniques are highly likely to be utilized in 100 gigabit transmission systems (100G), there is a general need for an effective form of polarization processing of transmitted signals so as to provide for electric field phase and amplitude detection and decoding. The utilization of polarization multiplexed phase encoding in 100G coherent systems allows for higher data rates of transmission. Detection of the electric field vector of the optically transmitted signal is particularly advantageous as it permits the calculation and mitigation of many transmission impairments and distortions such as chromatic dispersion induced pulse spreading and polarization mode dispersion.
The coherent transmission system is likely to rely on a dual polarization with a quadrature phase shift key modulation scheme (DP-QPSK). This is known to be especially efficient and provides improved signal to noise and allows for utilization of CMOS electronic decoder systems. Other forms of optical encoding are known and are also applicable with the present invention.
Turning initially to FIG. 1, there is illustrated one form of reference design 10 for a DP-QPSK transmitter. The reference design illustrates transmission on one wavelength band only. It will be obvious to those skilled in the art that it can be combined via multiplexing with other transmitters for other wavelength bands. In the DP-QPSK transmission system, an input laser 11 of predefined frequency and polarization state is input 12 and is interconnected to a number of modulators 13-16 which act under electronic control of drivers 17-20. The modulators 13-16 act to phase modulate the signal in a known controlled manner.
The modulators act to phase encode an input data stream. Polarization multiplexing is then provided by polarization rotation element 21 which outputs an orthogonal polarization 23 to second polarization 22. The two orthogonal polarizations are combined 25 by beam combiner for transmission.
The signal is then transmitted in a particular wave length band. During transmission, the orthogonality of the polarization states is substantially maintained although rotation of the overall polarization state may occur.
The receiver is then responsible for decoding the transmitted signal so as to extract the data information that has been encoded by the transmitter.